Question O2 Splitoff from "ScubaPro G260 or S620Ti?"

[EricSm]

Makes total sense. I am curious about nitrox. You mentioned that titanium is usually NOT considered stable when used with pure oxygen at a pressure higher than 2atm. But what about ppO2 in mixed gas? I know some particles and oils could act as catalysts for the titanium oxidation, making the reaction faster and exothermic -> dangerous. This is why we use special greases for oxygen-compatibility.

But even with a simple nx32, the ppo2 in your example would be pretty high: 3,5 ATM. Are oxy-compatible lubricants enough to make titanium regulators safe? If yes, how?

PS I know there are rules about oxygen compatibility which define the maximum % of oxy in a mix - but I don't remember them now, and also I am more interested in the chemistry here

Oxygen compatible greases don't make a titanium regulator safe in a ~100% environment. The O2 compatible greases are used because hydrocarbon based greases in combination with O2 form a shock sensitive explosive. The O2 compatible greases are generally flouropolymers e.g. PTFE or Teflon based. They are extremely toxic if combusted, but if that is occurring you have problems anyway.

Titanium forms a passivation layer to provide its corrosion protection. Aluminum and nickel are also passivated by an oxide layer, but the oxide layers are different. There is a relationship that describes this but basically Ti is very reactive so once its passivation layer is breached by mechanical impact, rubbing (e.g. oring and valve stem), etc. the O2 combusts the Ti faster than the passivation layer can form to snuff the reaction. Chromium, nickel and even aluminum based alloys are relatively less reactive and more prone to reforming the passive oxide layer.

The reactivity of Ti in oxygen goes down significantly as the concentration goes down. From memory it is something like 350 psi can combust Ti in 100% O2 in a static environment (without impacts etc) but in 45% O2 you need 2200 psi or so. From memory so don't hold me on that, but it is roughly correct.

I’m way out of depth here, and this is a great write up (and I don’t intend to use Ti for any o2 applications, I’m sold )
But, I wanted to follow up on this 2nd atage barrel part:

Wouldn’t the (relative) IP of 9-10bar at 2nd stage inlet, reducing to ambient at the mouthpiece (ie relative 1 atm) cause immense adiabatic cooling?

Probably it’s (part of) why Ti regs get away with (recreational) nitrox

(My brain is trying to brain late at night, it aint pretty)

The Ti regulators aren't the ones rated for 40% or more O2. At lower concentrations of O2 the chance of ignition is dramatically less (ignition pressure is above the normal working pressure of the second stage). Using appropriate cleanliness and compatible greases.

The admittedly off topic question of what regulator to use in 50% or 100% O2 - definitely not anything with Ti... The scubapro R195 or Apex or Diverite for up to 100% O2 don't use Ti.

I've seen... used a normal G260 regulator with 50% or 100% O2 and should not have. If someone had a titanium regulator and did that... not a good idea.

 

[ginti]

Oxygen compatible greases don't make a titanium regulator safe in a ~100% environment. The O2 compatible greases are used because hydrocarbon based greases in combination with O2 form a shock sensitive explosive. The O2 compatible greases are generally flouropolymers e.g. PTFE or Teflon based. They are extremely toxic if combusted, but if that is occurring you have problems anyway.

Titanium forms a passivation layer to provide its corrosion protection. Aluminum and nickel are also passivated by an oxide layer, but the oxide layers are different. There is a relationship that describes this but basically Ti is very reactive so once its passivation layer is breached by mechanical impact, rubbing (e.g. oring and valve stem), etc. the O2 combusts the Ti faster than the passivation layer can form to snuff the reaction. Chromium, nickel and even aluminum based alloys are relatively less reactive and more prone to reforming the passive oxide layer.

True, but aluminium is also NOT considered stable enough to work with pure oxygen. The only one stable enough is stainless steel due to the stability of oxydation layers of chromium and nickel. Anyway, not relevant for the sake of this discussion.

The reactivity of Ti in oxygen goes down significantly as the concentration goes down. From memory it is something like 350 psi can combust Ti in 100% O2 in a static environment (without impacts etc) but in 45% O2 you need 2200 psi or so. From memory so don't hold me on that, but it is roughly correct.

The Ti regulators aren't the ones rated for 40% or more O2. At lower concentrations of O2 the chance of ignition is dramatically less (ignition pressure is above the normal working pressure of the second stage). Using appropriate cleanliness and compatible greases.

The admittedly off topic question of what regulator to use in 50% or 100% O2 - definitely not anything with Ti... The scubapro R195 or Apex or Diverite for up to 100% O2 don't use Ti.

I've seen... used a normal G260 regulator with 50% or 100% O2 and should not have. If someone had a titanium regulator and did that... not a good idea.

This is actually what I am trying to understand. To me, intuitively, the ignition pressure should decrease together with concentration. For example, half of the concentration, double the ignition pressure; this way, the ppO2 would dictate the ignition.

But the way you are describing it, it seems to me that this is not true. E.g. nx32 (or even nx25) would lead to a ppO2 definitely higher than 2atm in the example you mentioned above (regulator tuned to 160psi). Even air would be borderline, but air is of course fine. Therefore, either the ignition pressure is higher than 3atm of ppO2, or, as you said, the concentration change the ignition pressure significantly for some specific reasons. Because you mentioned the latter is true, what are these reasons? Is nitrogen slowing the oxidative process? Or what?

I know I am asking quite a bit, so if you don't remember I will have a tour on Wikipedia (maybe guided by ChatGPT )

Thanks a lot for sharing your knowledge!

 

[rsingler]

Someone give me first-hand evidence of oxygen-related damage to a titanium SECOND stage. Any evidence. Anything at all.
Asking for a friend...

Here is a technical report.
NASA TR R-180 Reactivity of Titanium with Oxygen
The other technical report, one of the authors is Chaffey (of Grissom White and Chaffey Apollo 1 catastrophe).
The report states that in 100% gaseous oxygen they needed at least 350 psi for combustion to occur statically, but once it started, everything (metal etc) was consumed.

I don't want to get into a fight with you, @EricSm , I really don't.
I agree: Ti regs are clearly incompatible with 100% oxygen use. But the ONLY experimentation with titanium in gaseous oxygen in your citation above reads as follows:

The relation of these experimental conditions to a second stage where (as pointed out by others above) there is substantial adiabatic cooling, is miniscule.

Later, you commented that titanium will spontaneously combust in 350 psi oxygen, but the required temperature was not noted. It's not as simple as undisturbed exposure to oxygen.
Again, I'm not trying to stir the pot. I'm just saying that it's more than just the partial pressure of oxygen. How else could we use 40% oxygen at 200 bar (partial pressure of oxygen 80 bar!!) with a titanium first stage on a full tank? And then dropping to 10 bar at the second stage makes the likelihood of an issue at the second stage even smaller.

No one who has done the experiment will report it, lest a tech diver use his titanium second stage with 100% O2. Scuttlebutt has it that Atomic was able to burn a titanium first stage with some experiment that may or may not have included particulates or impact or adiabatic heating. Were they able to light up a second stage at 10 bar? I'm willing to bet, "No", due to adiabatic cooling, relatively slow gas flow, and difficulty of impact ignition. No one will ever tell.

Why am I dragging this out? Because I think that titanium has a whole set of voodoo surrounding it that just doesn't apply to much of what we do. If the market were big enough to support the research, I bet someone could prove that deco on 50% O2 is safe with a titanium second stage, and maybe even higher concentrations. But the market is too small to afford the experiments, and liability is too high in the US to permit its use anyway. So the voodoo stands.

The only scary thing that I've seen first hand, is that unscrewing a tight titanium port plug, from a titanium body, in a dark room, will reveal tiny sparks!! No way I'm doing that around oxygen!! But the port plug didn't ignite.

Nitrox isn't oxygen, and we don't know how much N2 completely snuffs the reaction.
Therefore, I think your standalone comment that implied '350 psi of oxygen is enough to ignite titanium' was a touch alarmist. That's all I'm suggesting.

I've seen... used a normal G260 regulator with 50% or 100% O2 and should not have.

Why shouldn't a G260 have been used with 100% O2?

 

[ginti]

I don't want to get into a fight with you, @EricSm , I really don't.
I agree: Ti regs are clearly incompatible with 100% oxygen use. But the ONLY experimentation with titanium in gaseous oxygen in your citation above reads as follows:
View attachment 884504
The relation of these experimental conditions to a second stage where (as pointed out by others above) there is substantial adiabatic cooling, is miniscule.

Later, you commented that titanium will spontaneously combust in 350 psi oxygen, but the required temperature was not noted. It's not as simple as undisturbed exposure to oxygen.
Again, I'm not trying to stir the pot. I'm just saying that it's more than just the partial pressure of oxygen. How else could we use 40% oxygen at 200 bar (partial pressure of oxygen 80 bar!!) with a titanium first stage on a full tank? And then dropping to 10 bar at the second stage makes the likelihood of an issue at the second stage even smaller.

No one who has done the experiment will report it, lest a tech diver use his titanium second stage with 100% O2. Scuttlebutt has it that Atomic was able to burn a titanium first stage with some experiment that may or may not have included particulates or impact or adiabatic heating. Were they able to light up a second stage at 10 bar? I'm willing to bet, "No", due to adiabatic cooling, relatively slow gas flow, and difficulty of impact ignition. No one will ever tell.

Why am I dragging this out? Because I think that titanium has a whole set of voodoo surrounding it that just doesn't apply to much of what we do. If the market were big enough to support the research, I bet someone could prove that deco on 50% O2 is safe with a titanium second stage, and maybe even higher concentrations. But the market is too small to afford the experiments, and liability is too high in the US to permit its use anyway. So the voodoo stands.

The only scary thing that I've seen first hand, is that unscrewing a tight titanium port plug, from a titanium body, in a dark room, will reveal tiny sparks!! No way I'm doing that around oxygen!! But the port plug didn't ignite.

Nitrox isn't oxygen, and we don't know how much N2 completely snuffs the reaction.
Therefore, I think your standalone comment that implied '350 psi of oxygen is enough to ignite titanium' was a touch alarmist. That's all I'm suggesting.


Why shouldn't a G260 have been used with 100% O2?

Same view of you so far.

Nevertheless, it is true that, at least officially (read it: liability issues), titanium regulators are not suggested/approved for oxygen operations. Also, I really appreciate the knowledge that @EricSm is sharing.

I still believe the topic to be complicated enough and deserving further explanations by those who know enough (not me)

 

[rsingler]

Also, I really appreciate the knowledge that @EricSm is sharing.

Me too. He clearly knows the subject well.

 

[Geo7]

There are aluminum regulators certified for use with pure oxygen:

Aluminum Regulators Suitable for Oxygen

 

[EricSm]

Why shouldn't a G260 have been used with 100% O2?

Simply because it isn't "rated" for 100% O2. The Scubapro R195 regulator for 100% O2 (that they don't sell in the US) uses the same parts from what I've been told as a regulator rated to less than 100%. For example Nitrile Orings and silicone bladder, O2 compatible grease.
There are system design considerations for oxygen systems (and assemblies) for example, how quickly the system can be pressurized by use of slow opening valves (found on O2 bottles first stages), what can be impacted by a contamination particle downstream of a flow restriction, screens to filter particulates, etc.

 

[rsingler]

Simply because it isn't "rated" for 100% O2. The Scubapro R195 regulator for 100% O2 (that they don't sell in the US) uses the same parts from what I've been told as a regulator rated to less than 100%.

I interpret this to mean "bureaucratic decision making", or "limitation of liability" rather than a scientific reason to avoid it. Your second sentence confirms this.

It is a straightforward matter of companies choosing not to incur the expense of certifying under European regulating norms EN13949 and EN 144-3.

G260: stainless steel orifice; non-titanium barrel; non-titanium poppet spring; able to be oxygen cleaned and lubricated with oxygen safe lubricant; adiabatic cooling at the orifice; low gas velocity in comparison to the first stage.
My  personal conclusion is that I personally would have no hesitation in doing accelerated deco with a G260 on 100% O2. I cannot speak for anyone else. The practical matter is that using an expensive reg like a G260 on a slung bottle for 20 min of accelerated deco is kind of silly compared with using an R195.
Then again, using a Mk25 EVO for the same thing is also silly, just because of the EN certification, when there are inexpensive Dive Gear Express firsts sold as oxygen ready.

I'll expand my earlier question: someone give me first-hand evidence of oxygen-related damage to ANY second stage. Any evidence of combustion that did not start upstream. Anything at all.
Asking for a friend...

 

[EricSm]

True, but aluminium is also NOT considered stable enough to work with pure oxygen. The only one stable enough is stainless steel due to the stability of oxydation layers of chromium and nickel. Anyway, not relevant for the sake of this discussion.
There are a whole bunch of materials that are compatible with oxygen systems. Pretty much the only one that is compatible with high pressure (up to 10,000 psi) 100% O2 is Nickel 200. Think of a sintered metal filter made of pure nickel. It can withstand "dirt" particles in a 100% O2 flowstream. The NASA Safety Standard for Oxygen and Oxygen Systems lists bunch of materials and the conditions in which they are suitable for use. I would be surprised if the folks that are designing regulators and rebreathers were trained in oxygen system design and materials. It worries me a bit.


This is actually what I am trying to understand. To me, intuitively, the ignition pressure should decrease together with concentration. For example, half of the concentration, double the ignition pressure; this way, the ppO2 would dictate the ignition.

But the way you are describing it, it seems to me that this is not true. E.g. nx32 (or even nx25) would lead to a ppO2 definitely higher than 2atm in the example you mentioned above (regulator tuned to 160psi). Even air would be borderline, but air is of course fine. Therefore, either the ignition pressure is higher than 3atm of ppO2, or, as you said, the concentration change the ignition pressure significantly for some specific reasons. Because you mentioned the latter is true, what are these reasons? Is nitrogen slowing the oxidative process? Or what?

The O2 safety literature deals with ignition pressures and temperatures. There is a certain amount of energy that needs to be put into a system containing 100% O2 to initiate the combustion process (the fire triangle). In the fire triangle heat, fuel and oxidizer, you have 100% o2 so lots of oxidizer, pretty much everything is a fuel so you just need heat or energy to light it off. At lower oxygen concentrations the energy required to sustain or initiate combustion is greater. The stochiometric fuel and oxidizer ratio falls out of balance with nitrogen or other inerts absorbing heat but not oxidizing and contributing heat to sustained combustion.

Here are some examples of ignition energies required to ignite stuff in a 100% O2 atmosphere:

Buna-N (Buna Bad) has a very low energy requirement to initiate combustion and has a high heat of combustion so will sustain a fire one initiated. I've seen old salts poo poo the need to use O2 compatible o-rings. I wish there was better O2 safety training.

Nitrile - requires just 22 ft*lbs (29.8) joules in gas and only 10 ft*lbs in liquid O2 to combust. Think of whacking open an O2 valve and the energy imparted to the Oring.

Viton is a good elastomer seal material and is used in O2 systems to 6000-7000 psi.

Particle impact is the most efficient way to ignite metals e.g. aluminum oxide from inside an aluminum tank or iron oxide in a steel tank is accelerated through a first or second stage (drop in pressure=increase in velocity), that entrained particle impacts something with a relatively low ignition energy. In properly designed oxygen systems the nearest bend is x number of 1/2 wavelengths from the orifice to avoid particle impact or resonance. I doubt this is evaluated in rebreather design (lot of home brew stuff that got commercialized) or even big name recreational SCUBA regulators. I say this because anyone that sat through O2 system training or read that NASA technical spec (NSS 1740.15 available free, first result in google search. It is a good read) would have selected any one of the other more appropriate materials (eg Inconel, brass, Monel, Hastelloy, etc). I love Titanium, wrote a report on the element in middle school. Titanium Grade 5 is one of the best materials to use subsea, but it doesn't belong in oxygen systems.

Oxygen systems must be filtered to 10-25 microns upstream of a regulator. Typical brass oxygen welding regulators filter to 50-100 microns. There are loads of examples and pictures of brass oxygen regulators (first stage) that have burned themselves up and the building it is housed in. Brass is a great material for O2 systems (even used in high pressure), but rapidly opening an O2 valve or presence of hydrocarbon grease or a filter that fell off inside the tank have resulted in lots of accidents.

Teflon or PTFE is one of the best materials for use in O2 but if combusted gives off carbonyl flouride, which is lethal at 3 ppm and has an 8 hour exposure limit of 2 ppm.

Happy to share anything relevant from what I picked up. I was the system engineer for a deep subsea PEM fuel cell project based on stored O2 and H2 at 5000 psi. The oxygen and hydrogen safety training we commissioned from some ex NASA folks was enlightening/alarming. NASA had lots and lots of problems with oxygen in the early days. There is little appreciation for the hazards of pure o2 in scuba both with oxygen blending stations and the use of pure o2 in rebreathers and technical diving. In December in Bonaire I had a 100% O2 deco bottle, the O ring fell off the first stage and an old salt was insisting that any O ring was fine. Found the right one and didn't argue, but somehow there needs to be better training.

I know this has gotten way off topic, but Titanium in enriched O2 is bad. Blended tanks often are poorly mixed initially so the EAN 32 or 40 or whatever often has stratified gas for awhile (typically air on top though). It isn't difficult to plop your regulator on somebodies 100% O2 accelerated deco bottle (that should have been analyzed first).

I know I am asking quite a bit, so if you don't remember I will have a tour on Wikipedia (maybe guided by ChatGPT )

Thanks a lot for sharing your knowledge!

 

[Pearlman]

Pretty sure the OP is now frightened and discouraged from even attempting SCUBA as sport … (“All I wanted was a recommendation for a first reg … ”) and the discussion veered off towards NASA and space level tech…